The invention relates to a process for producing lithographic printing forms from grained, anodized and hydrophilized lithographic printing plates which have a radiation-sensitive coating, are exposed and are developed in an aqueous alkaline solution, and to lithographic printing forms produced thereby.
The presensitized lithographic printing forms generally used in the field are metal, usually aluminum, bases in strip, plate or sheet form which are provided with positive- or negative-working radiation-sensitive
The following radiation-sensitive coatings are normally used to coat the printing plates:
positive- or negative-working diazo compounds, PA0 negative-working mixtures composed of olefinically unsaturated compounds and photoinitiators, and PA0 positive-working mixtures with photosemiconductors.
As film formers, these coatings contain polymers having functional units which ensure the development of the coatings after the imagewise exposure and optional further processing steps, such as heat treatment or toning, in the aqueous alkaline developer solutions. Examples of such functional units are --COOH, --SO.sub.3 H, --PO.sub.3 H.sub.2, --SH, --OH and --NH.sub.2.
As alkaline components, the developer solutions generally contain, in addition to further constituents, such as surfactants, hydrotropes, solvents, complexing agents etc., alkali-metal silicates. Silicates have the advantage that they attack the Al.sub.2 O.sub.3 layer built up in the anodizing step on the surface of the base to a markedly lesser extent than other alkaline reagents.
Nevertheless, problems occur in practice, and specifically, of a type, in particular, which is such that the Al.sub.2 O.sub.3 formed on the front and back of the aluminum base is subject to a degradation in the development process which is in some cases severe. On the back of the aluminum base, in particular, which has only a relatively thin oxide coating, this results in a gelatinous coating which accumulates on the squeeze rollers of the development apparatus and may be retransferred from that point to the developed lithographic printing forms. In order to avoid the malfunctions in the printing process resulting therefrom, prevention of this contamination by regular cleaning of the development apparatus is therefore necessary.